Suggest using .git-blame-ignore-revs when adopting ktfmt

Git has [`--ignore-revs-file`](https://git-scm.com/docs/git-blame#Documentation/git-blame.txt---ignore-revs-filefile) option that allows skipping particular commits during `git blame`.

By convention, GitHub expects a root `.git-blame-ignore-revs` file to be used as `--ignore-revs-file`, as well as GitLab, Black [also] recommends using it.

It would be nice to recommend this approach in the README, both for formatter adopters and for backwards-incompatible changes in the formatter (e.g. #293)

Start in the README, where formatter adoption and backwards-incompatible changes are documented. Add a recommendation to use a root .git-blame-ignore-revs file with Git blame, covering both adopting ktfmt and formatter changes such as #293; done means the README clearly explains this workflow.

Written by the indexing model from the issue text.